The activity series is a series which indicates the positions of various elements and their ability to displace each other from water or acids.

Hydrogen as a peculiar element can be displaced from water and acids by some metals. Metals which displace hydrogen from its solution are more electropositive (more reactive) than hydrogen while those that cannot displace hydrogen are less electropositive.

The arrangement of metals in the electrochemical series is due to their increasing electropositivity. Sodium and potassium reacts vigorously with cold water to liberate hydrogen, calcium reacts slowly with cold water and more rapidly with steam to liberate hydrogen gas, Magnesium, Aluminum, Zinc and Iron react with steam to liberate hydrogen gas. copper and lead do not react at all with water.

Considering the displacement of metals in solution, when a piece of iron rod is dipped into a solution of copper (II) tetraoxosulphate (VI), reddish brown deposits of copper metal are formed on the iron rod after sometime. Hence, iron can displace copper from a solution of copper (II) salt.

The following is the activity series of metals according to their increasing electropositivity and decreasing chemical activity.

1. Lithium
2. Potassium
3. Sodium
4. Calcium
5. Magnesium
6. Aluminum
7. Zinc
8. Iron
9. Nickel
10. Tin
11. Lead
12. Hydrogen
13. Copper
14. Mercury
15. Silver
16. Gold

1. Pure hydrogen is odourless, colourless and tasteless.
2. It is neutral to litmus paper.
3. It is relatively insoluble in water.
4. It is the lightest known substance, about 14.4 times less dense than air.
5. It has a very low boiling point of about -253°C

1. Reaction with oxygen: Hydrogen burns with a pale blue flame in oxygen to form steam $$ 2H_2 + O_2 \rightarrow 2H_2O $$
2. Reaction with halogens : Hydrogen combines directly with halogens to form halides. $$ H_2 + F_2 \rightarrow 2HF$$ $$ H_2 + Cl_2 \rightarrow 2HCl $$ With chlorine, the reaction is more rapid in bright sunlight and slower in diffused light.
3. Ability to accept electrons: Hydrogen accepts electrons to form hydride ion (H^-) $$ 2Na + H_2 \rightarrow 2NaH $$
4. Combination reactions
   1. With metals: Hydrogen combines directly with metals to form ionic hydride. $$ Ca + H_2 \rightarrow CaH_2 $$
   2. With Nitrogen: Hydrogen combines reversibly with Nitrogen in the presence of Platinum as catalyst under standard conditions to form ammonia $$ 3H_2 + N_2 \rightleftharpoons 2NH_3 $$
5. Reducing action: Hydrogen serves as a strong reducing agent. It reduces oxides of metals to the corresponding metal. $$ PbO + H_2 \rightarrow Pb + H_2O $$ $$ CuO + H_2 \rightarrow Cu + H_2O $$ $$ Fe_2O_3 + 3H_2 \rightarrow 2Fe + 3H_2O $$

Insert a lighted splinter into a test tube containing the unknown gas. The pop sound indicates the presence of hydrogen gas.

1. Due to its low density, it can be used for filling balloons.
2. Liquid hydrogen serves as rocket fuel.
3. Hydrogen is used in making important chemicals like ammonia, hydrochloric acid, ethanol, etc.
4. Hydrogen in the presence of Nickel as catalyst is used to harden animal or vegetable oil to margarine.
5. Hydrogen is useful in the manufacture of soaps.
6. It is used in oxy-hydrogen flames which produces high temperatures or heat that can be used to melt metals.
7. Hydrogen is useful in the production of atomic hydrogen flames.

1. Hydrogen or protium, \(^1_1 H \)
2. heavy hydrogen or deuterium, \(^2_1 H \)
3. Tritium, \( ^3_1 H \)
The most abundant isotope is protium, although it is leas reactive. Deuterium forms an oxide, \( D_2O \) called heavy water. Tritium is radioactive with a half-life of about twelve years.

Metals like potassium, sodium and calcium forms ionic hydrides with hydrogen. These hydrides share some properties with other ionic compounds. The hydrides have high melting and boiling point. They react with water to form hydroxides and are good conductors of electricity.

Boron and aluminum form covalent hydrides which are very good reducing agent in organic chemistry e.g Lithium tetrahydrido aluminate (III), LiAlH₄ and sodium tetrahydrido borate (III), NaBH₄.

Non metals such as chlorine and nitrogen form simple covalent hydrides e.g. HCl, NH₃, HF.

Water is an exception because they are liquids due to hydrogen bond in them.